Autonomous vehicles, such as vehicles that do not require a human driver, can be used to aid in the transport of passengers or items from one location to another. Such vehicles may operate in a fully autonomous driving mode where passengers may provide some initial input, such as a destination, and the vehicle maneuvers itself to that destination. Thus, such vehicles may be largely dependent on systems that are capable of determining the location of the autonomous vehicle at any given time, as well as detecting and identifying objects external to the vehicle, such as other vehicles, stop lights, pedestrians, etc. As an example, these systems may include sensors, such as laser scanning equipment and cameras, mounted at various locations on the vehicle.
While such sensors come in many different configurations, as an example, a (“light detection and ranging”) LIDAR sensor may include at least a laser light source, an optical sensor, and a controller. The laser light source provides a pulsed beam of light into the environment of the LIDAR sensor. Any light from the beam of light that is reflected off of an object back towards the LIDAR sensor is detected by the optical sensor. The distance from the reflecting object is determined by the controller and a record of the surface is recorded as an object in space. The sensor data from successive measurements can be used to generate a 3D map of the environment. The usefulness of such maps is dependent upon the sensor and laser having an unobstructed view through the sensor windows.
For instance, an area of the sensor window may become contaminated with dust, dirt, mud, salt, leaves, water, or other substances or debris that act as an optical interferent. In this regard, the optical interferent interferes with the path of the beam of light from the laser light source away from the sensor window and/or the path of reflected light from the beam of light back through the sensor window towards the optical sensor. Thus, the optical interferent may cause the LIDAR sensor to generate incorrect information for the area of the sensor window.
As a result, the performance of the LIDAR sensor can be either degraded or the data generated by the LIDAR sensor (sensor data) can become completely invalid. For example, an opaque optical interferent on the sensor window will attenuate the beam of light, blocking the function in the affected scan area. Water or other transparent liquid may not block the beam of light completely, but water may act like a lens and deflect the beam of light. Large-angle deflection has the effect of reducing the intensity of returning or reflected light, similar to the opaque optical interferent. Smaller-angle beam deflection can return incorrect data to the optical sensor, resulting in valid-looking measurements from objects in locations different than where the LIDAR sensor is actually aiming the beam of light. In some cases, the sensor data from the deflected light degraded to the point that the noise of the LIDAR sensor overwhelms the desired sensor data. Thus, the sensor data becomes useless and unusable by vehicle's computing devices, creating potentially dangerous situations.